The invention relates to a process for the preparation of amines in which at least one substient is aromatic and is monosubstituted to trisubstituted by halogen. In this connection, the process starts out from reaction products of oxo compounds and amines or ammonia which carry halogen substituents in the circumstances mentioned. These reaction products are catalytically hydrogenated, an Ni-containing and/or Co-containing catalyst being employed and the reaction being carried out in the presence of organic sulphur compounds.
The catalytic hydrogenation of reaction products of oxo compounds and amines or ammonia is known. Aldehydes and ketones may be mentioned as oxo compounds in accordance with the following scheme: ##STR1##
From an aldehyde or ketone and a primary or secondary amine or ammonia, a hemiaminal can therefore first be formed which can react further with a further molecule of the amine or ammonia to give the aminal with the elimination of a molecule of water or (only with primary amines or ammonia) can also react to give the azomethine (Schiff's base, aldimine, ketimine) with the elimination of a molecule of water. Aldimines can additionally be obtained in an indirect manner as reaction products of aldehydes and ammonia by partial hydrogenation of nitriles. All reaction products mentioned (hemiaminal, aminal or azomethine) can be catalytically hydrogenated to give the corresponding amines (Houben-Weyl, Methoden der Organischen Chemie (Methods of Organic Chemistry), 4th edition, volume IV/lc (1980), p. 127/128, 239/240 and 436). Such catalytic hydrogenations should also be possible with the retention of halogen. However, this result is limited to the use of palladium catalysts, and it is further pointed out that the reaction is advantageously carried out at low temperatures and using a deactivated catalyst (page 240). Even using other catalyts, such as platinum or Raney nickel, halogen should be retained (Houben-Weyl, loc. cit.). However, the specific hydrogenations indicated in the references cited do not represent a systematic investigation (page 436, paragraph 3) and in some cases show extremely moderate yields, as in the case of p-chlorobenzyl methyl ketone which can only be converted into the respective amine in 10% of the theoretical yield (Houben-Weyl, bottom of page 436). By-products have to be taken into account, especially in the preparation of strongly basic amines (Houben-Weyl, age 240, paragraph 2).